Imaging radiation detectors, for example for X-rays or electrons, can take the form of a hybrid, in which a read-out chip includes electronics for reading out and processing data, and a sensor is mounted directly over the read-out chip, this assembly having a number of pixels resulting from a regular grid of interconnection bonds between the sensor and the read out chip (planar geometry). In an alternative setup the sensor itself has pixels due to a regular arrangement of electrodes machined into the sensor (3D) geometry). The pixels are to detect X-rays, electrons or generally ionizing radiation, spatially resolved, arriving at the respective pixel. When an X-ray, electron or generally ionizing radiation interacts with a pixel, it creates electric charge which is read out by the read-out chip. Such detectors can act as imaging sensors.
A commercial radiation detector of the first type is the PIXcel (trade mark) detector sold by Panalytical BV which uses a Si sensor. Each pixel of the sensor has a respective bump bone connecting the pixel to a respective read-out circuit.
When a X-ray is absorbed in a pixel of the sensor it interacts with an atom to produce a photoelectron that in turn excites a number of outer electrons from Neighbouring atoms an hence creates a cloud of electrons (and holes) in a small spatial region of the sensor material. There may be of order a couple of thousand electrons in the cloud. The number of electrons is proportional to the energy of the X-ray photon. The bias voltage causes the electron could to diffuse to the back of the sensor where it is passed through the bump bond to the read-out chip and is transformed into an electrical pulse by the individual circuit of the pixel and detected as a count.
The read-out chip can process the electrical pulse received to build up an image. This can be done in a number of ways. Usually counts are collected, spatially resolved, in the pixel matrix of the detector once a generated electrical pulse exceeds a voltage threshold. The different amounts of counts collected in the pixel matrix form an absorption contrast pattern, i.e. an image of an object. In X-ray diffraction experiments the counts of pixels form 2-dimensional X-ray diffraction patterns and they can also specially be integrated or binned together to form diffractograms, the latter being realized with the PIXcel detector.
The use of germanium in a radiation detector is taught by U.S. Pat. No. 6,933,503 which describes a bulk germanium X-ray detector sandwiched between two thin oppositely doped GaAs layers which accordingly constitute a pn diode. One of the GaAs layers is divided to make a plurality of pixels. The germanium is much thicker than the GaAs layers so that the X-rays are substantially all absorbed in the bulk germanium.
X-rays or electrons can cause damage to the read-out chip when they interact with the read-out chip. This can cause problems and failure of the radiation detector. Such issues are a particular problem in the case of laboratory X-ray analysis using Mo radiation at 17.4 keV or using X-rays from a synchrontron source at up to 20 keV instead of the more typical Cu radiation at 8 keV for detectors when being equipped with a Si sensor.
A known approach to reducing the damage caused by X-rays or electrons is to use a material that inherently absorbs more X-rays or electrons such as GaAs, CdZnTe or CdTe instead of Si. Unfortunately, such materials are still very problematic as sensors and normally give limited detector performance. Processing on Si is normally much easier than with such more exotic materials.
Another approach is that taught in U.S. Pat. Nos. 5,889,313 and 6,204,087 which teach a different configuration with anode bars extending through a sensor. These bars are biased to collect electrons. The configuration has the advantage that the electrons generated by radiation have less far to travel to the anode bars than they would to a collection or measuring electrode on the surface of the sensor. The configuration is said to reduce radiation damage.
However, this configuration has proven very difficult to manufacture in practice to achieve commercial devices. It is very difficult to manufacture very narrow through-holes in a 300 μm thick substrate with the correct aspect ratio and spacing. If a thinner sensor is used, absorption is generally too low for optimal sensitivity as is typically the case for X-ray laboratory analysers.
Thus, there remains a need for a Si-based sensor that better protects the read-out chip against radiation damage in the range 8-20 keV. This is important because read-out chips of high spatially resolving detectors may be fabricated in sub-micron CMOS processes which show a certain radiation hardness but do not employ specifically radiation hard designs.